This invention relates generally to hydraulic actuators and more particularly to motion control for such actuators.
Aircraft powerplants are typically used to drive thrust-generating airfoil elements such as propellers or fan blades. It is known to vary the angle of incidence (i.e. “pitch angle”) of the airfoil elements relative to the rotating hub carrying them, in order to provide the maximum possible propulsive efficiency at various flight conditions.
A common method of pitch control employs a hydraulic actuator which changes the blade pitch angle in response to pressurized fluid flow. The actuator may move the blade through pitch angles from “coarse” to “fine” and may also provide pitch angles suitable for ground operation. For safety reasons, it is important to limit the blade pitch angle during flight. This avoids overspeeding the powerplant, or imposing excessive structural loads or unexpected yawing moments to the aircraft. A typical prior art variable-pitch propeller includes a mechanical pitch stop or lock which limits the blade pitch angle and must be manually retracted in order to move the blades towards positions in the ground operating range.
While mechanical pitch stops are effective, they add complexity, weight, and cost to the basic actuator device. Accordingly, there is a need for an actuator which provides two ranges of rotary movement without a mechanical lock or stop to define the limit between ranges.